Networks of rhythmically active neurons in the spinal cord are responsible for producing locomotor movements. In developing vertebrates, ventral spinal cord can be sub-divided into five zones, four of which are responsible for producing the interneurons that drive rhythmic motoneuron activity (V0-V3). Critically, interneurons that emerge from these zones are labeled by the same transcription factors and have similar morphologies and functions in different vertebrates. This has made it even easier to compare features of circuit organization in simpler model systems, like fishes and frogs, to more complex ones, like chicks and mice. What is unclear, however, is how only four progenitor zones can yield the diverse array of functionally distinct interneurons that are known to exist. Our plan is to explore the contribution of development to the functional elaboration of spinal circuitry, by studying a genetically identified population of spinal interneurons in zebrafish. Our work suggests that interneurons labeled by the transcription factor alx in zebrafish are not functionally identical and contribute to different speeds of movement. However, it is not clear how differences in the morphology, connectivity and the excitability of alx interneurons may be related to their function and whether developmental programs shape these features. The remarkable conservation of developmental mechanisms leads us to think that the patterns we find will be present broadly among vertebrates, including mammals. By doing so, we hope to better explain and treat disorders that affect the speed and coordination of movements, like Parkinson's disease or spinal injury.